fix: UUID fallback for non-secure HTTP contexts

crypto.randomUUID() is unavailable outside secure contexts (plain HTTP).
Add generateUUID() helper with manual UUID v4 fallback (RFC 4122).

frontend/src/lib/uuid.ts

@@ -0,0 +1,18 @@
+/**
+ * Generate a UUID v4 string.
+ *
+ * Uses crypto.randomUUID() when available (secure contexts / HTTPS / localhost).
+ * Falls back to a manual UUID v4 generator for non-secure contexts (plain HTTP).
+ */
+export function generateUUID(): string {
+  if (typeof crypto !== 'undefined' && typeof crypto.randomUUID === 'function') {
+    return crypto.randomUUID()
+  }
+
+  // Manual UUID v4 fallback (RFC 4122)
+  return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, (c) => {
+    const r = (Math.random() * 16) | 0
+    const v = c === 'x' ? r : (r & 0x3) | 0x8
+    return v.toString(16)
+  })
+}

frontend/src/pages/LTTPage.tsx

@@ -7,6 +7,7 @@ import { useSystemAudioASR } from '../hooks/useSystemAudioASR'
 import { useMicASR } from '../hooks/useMicASR'
 import { useFullTranscript } from '../hooks/useFullTranscript'
 import { getVideoUrl } from '../lib/api'
+import { generateUUID } from '../lib/uuid'
 import { QueryInput } from '../components/QueryInput'
 import { ExtractedQuestionsDisplay } from '../components/ExtractedQuestionsDisplay'
 import { ResponsePanel } from '../components/ResponsePanel'
@@ -39,12 +40,12 @@ export const LTTPage: React.FC = () => {
   const systemAudioWsUrl = useMemo(() => {
     const protocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:'
     const host = import.meta.env.VITE_WS_HOST ?? window.location.host
-    return `${protocol}//${host}/ws/asr/${crypto.randomUUID()}?language=yue&source=system-audio`
+    return `${protocol}//${host}/ws/asr/${generateUUID()}?language=yue&source=system-audio`
   }, [])
   const micWsUrl = useMemo(() => {
     const protocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:'
     const host = import.meta.env.VITE_WS_HOST ?? window.location.host
-    return `${protocol}//${host}/ws/asr/${crypto.randomUUID()}?language=yue&source=mic`
+    return `${protocol}//${host}/ws/asr/${generateUUID()}?language=yue&source=mic`
   }, [])
 
   const systemAudioASR = useSystemAudioASR({ wsUrl: systemAudioWsUrl })